This application is based on an application No. H11-325790 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a flash-based image fixing apparatus that is used in an electrophotographic image forming apparatus such as a laser printer. In particular, the invention relates to an improvement in the supply of electricity to a flash lamp.
2. Related Art
Electrophotographic image forming apparatuses, such as laser printers, develop a latent image using toner, transfer the toner image onto a sheet of paper, and then fix the toner image using a fixing apparatus. Heated rollers are often used as this fixing apparatus, but take a long time to warm up. For this reason, increasing attention is being given to flash-based fixing apparatuses where a flash lamp is instantaneously illuminated to emit light (in particular infrared light) that supplies energy to melt the toner. Such toner includes a color agent, a binder, and the like.
FIG. 1 is a block diagram showing the circuit construction of a conventional flash-based image fixing apparatus.
As shown in FIG. 1, the flash-based fixing apparatus includes a flash lamp 200, a flash power supply unit 300, and a flash power supply control unit 500. The flash lamp 200 is filled with xenon gas. The flash power supply unit 300 supplies power to the flash lamp 200. The flash power supply control unit 500 controls the flash power supply unit 300. The flash power supply unit 300 includes an AC-DC convertor 310, a diode 320, a power-supplying film capacitor 330, a trigger circuit 340, and a choke coil 350. Of these, the diode 320 prevents reverse currents, the power-supplying film capacitor 330 has a capacitance of around 200 xcexcF, and the choke coil 350 suppresses the discharge current.
In accordance with a charging indication given by the flash power supply control unit 500, the AC-DC convertor 310 converts an AC voltage, supplied by the commercial power supply 800 via the power supply switch 900, into a DC voltage of around 2000 V. This DC voltage is supplied to the film capacitor 330 via the diode 320.
When the voltage across the terminals of the film capacitor 330 reaches a charging stop voltage, such as 2000 V, the flash power supply control unit 500 instructs the AC-DC convertor 310 to stop the charging of the power-supplying film capacitor 330.
At the point where charging is stopped, an amount of electrostatic energy ((0.0002*20002)/2=400J) that is required to melt toner on a piece of paper across a predetermined width has accumulated in the film capacitor 330. Two thousand volts are applied across the main electrodes of the flash lamp 200 via the choke coil 350 as the charging stop voltage of the film capacitor 330. However, the flash lamp 200 does not illuminate as long as a trigger signal is not inputted.
This charging stop voltage is set at a value that exceeds the minimum voltage (such as 1200 to 1500 V) required to initiate discharge (i.e., illumination) within the flash lamp 200.
At a predetermined time following this, the flash power supply control unit 500 instructs the trigger circuit 340 to start the illumination of the flash lamp 200. The trigger circuit 340 includes a capacitor (not illustrated) for generating a trigger signal. On being instructed by the flash power supply control unit 500 to illuminate the flash lamp 200, the trigger circuit 340 discharges this capacitor and outputs the resulting voltage to the trigger electrode of the flash lamp 200 as the trigger signal. Once the trigger signal has been inputted into the trigger electrode, the electrostatic energy that has accumulated in the film capacitor 330 is released so that a discharge current flows though the main electrodes of the flash lamp 200. As a result, the flash lamp 200 momentarily illuminates due to arc discharge, for a period of between several hundred microseconds to one millisecond.
FIG. 2 shows the transition in the discharge current that flows through the flash lamp during the illumination period.
As shown in FIG. 2, the discharge current that flows through the flash lamp 200 falls from an initial peak of 300A to virtually 0A during the illumination period of around one millisecond. In more detail, during the initial stage (shown as xcex41) in the illumination period, the discharge current hits a peak of 300A at roughly the same time as the trigger signal is applied and thereafter drops to 130A. In a short second stage (shown as xcex42), the discharge current stabilizes at around 130A. In the third and final stage (shown as xcex43), the discharge current falls from 130A to 0A.
As described above, the flash lamp 200 is repeatedly illuminated to supply energy that melts the toner on the paper, thereby fixing the toner image to the paper.
As one example, When an image is formed using black toner, the coloring agent, such as carbon black, that is located near the surface of the toner particles absorbs the light energy produced by the flash lamp 200 and converts it into heat energy. This heat energy is transmitted from the surface of the toner particles to the center. As a result, the entire toner particles melt, which fixes the toner to the paper. Here, it is preferable of the illumination of the flash lamp 200 to be prolonged so that the heat energy can be properly transmitted into the centers of the toner particles.
In a conventional flash-based fixing apparatus, the discharge current that flows through the flash lamp 200 is suppressed by the choke coil 350 to extend the illumination period to around one millisecond. However, further extension of the illumination period is difficult due to the occurrence of ringing in the circuit. Also, in the first stage of the illumination (shown as xcex41 in FIG. 2), there is a sudden drop in discharge current from 300A to 130A, during which time the majority (up to around 83%) of the static charge that has accumulated in the film capacitor 330 is used up.
To extend the illumination period in a conventional flash-based fixing apparatus, it is only possible to increase the amount of static energy supplied to the flash lamp 200. This raises the peak of the current value in the initial stage of the illumination period, and makes the illumination energy produced in this initial stage extremely high. This destroys the balance between the speed at which the coloring agent is heated and the transmission of the heat to the center of the particles, resulting in sublimation of the toner particles.
Conversely, if the amount of static energy supplied to the flash lamp 200 is reduced, the peak in the discharge current will be lowered, which reduces the sublimation of toner. However, if the illumination period is shortened, less heat energy is produced, so that the toner cannot be properly fixed. With a conventional flash-based fixing apparatus, therefore, improving the adhesion of the toner to the paper results in a great loss of toner through sublimation. In addition to this loss, sublimation also generates noise and unpleasant smells, and so is a first problem for conventional apparatuses. When fixing an image made with color (i.e., non-black) toner, the coloring agent present in the toner is poor at absorbing infrared radiation, so that toners are produced so as to include an infrared absorbing agent, such as a cyanine compound that has an absorbance peak for radiation with a wavelength of 800 to 1100 nm. When the proportion of such infrared absorbing agent reaches 3 to 5% by weight, however, there is an unavoidable rise in the cost of the toner.
Also, to fix the toner using an illumination period of around one millisecond, is necessary to raise the charging stop voltage of the capacitor in the flash-based fixing-apparatus to increase the illumination energy several fold. As mentioned above, however, raising the illumination energy of the flash lamp 200 also raises the discharge current during the initial stage of the illumination period.
When images made with color toner(s) and black toner are superimposed on the same sheet of paper and both kinds of toner are fixed by the same fixing operation, the black toner absorbs an excessive amount of illumination energy that exceeds the amount absorbed when the charging stop voltage is 2000 V. This means that even more of the black toner is sublimated, leading to problems in the fixed image such as partial omissions and marks left by sublimated toner. This means that there is a second problem of an increase in the cost of producing images and the inability to fix color and black images at the same time.
In order to solve this second problem, it would conceivably be possible to raise the amount of infrared absorbing agent in the color toner to 5 to 10% by weight so that color toner could be fixed using the same amount of illumination energy as black toner. However, this would increase the cost of color toner and create another problem of the infrared absorbing agent adversely affecting the color of the color toner.
It is a first object of the present invention to provide a flash-based fixing apparatus that improves the bond between the toner and the paper while suppressing the sublimation of toner.
It is a second object of the present invention to provide a flash-based fixing apparatus that enables color toner to be fixed at the same time as black toner while suppressing the cost of color toner used.
The above first object can be achieved by a flash-based fixing apparatus for fixing toner on paper using energy provided by an illumination of a flash lamp, the flash-based fixing apparatus including: a power supply unit for supplying power to the flash lamp so that the flash lamp illuminates; and a control unit for having the power supply unit supply power to the flash lamp so that a current flowing through the flash lamp during illumination is approximately constant at a predetermined value.
With the stated construction, the control unit suppresses the amount of power to the flash lamp so as to keep the current flowing through the flash lamp roughly constant during illuminlation at a predetermined value. This evens out the rate at which light energy is produced when the flash lamp illuminates and prolongs the illumination period. While conventional apparatuses are constructed so that the discharge current is very high at the start of illumination, leading to the sublimation of toner, the present invention greatly suppresses the discharge current at the start of illumination, prolonging the illumination period and improving the fixing of the toner to the paper. This also suppresses the noise caused by sublimation.
The above second object can be achieved by a flash-based fixing apparatus for fixing toner on paper using energy provided by an illumination of a flash lamp, the flash-based fixing apparatus including: a power supply unit for supplying power to the flash lamp so that the flash lamp illuminates; and a control unit for controlling the power supply unit to supply power to the flash lamp in accordance with a condition that differs depending on a factor relating to the toner on the sheet.
With the stated construction, current is supplied to the flash lamp according to different conditions depending on factors relating to the toner. This makes it possible to change the amount of power supplied to the flash lamp depending on whether black toner or color toner is being fixed. The fixing of toner is therefore improved without increasing the cost of the color toner.